1. Field of the Invention
The present invention relates to an organic electroluminescence device used in a color display and the like, and a display unit using the organic electroluminescence device.
2. Description of the Related Art
In recent years, so-called organic electronics has been receiving attention. Organic electronics is technology related to organic electroluminescence (organic EL), organic solar cells, organic transistors, organic memories, and the like in which organic materials formed into thin films are used in displays, batteries, transistors, recording devices, and the like.
Among organic electronics, organic EL is receiving attention as a next-generation display technology. Specifically, in 1987, Dr. Tang and others of Eastman Kodak Company presented an organic electroluminescence device capable of low-voltage drive and high-intensity light emission. The organic electroluminescence device includes an organic layer between an anode and a cathode. The organic layer has a lamination structure including a hole transport layer, a light emitting layer, an electron transport layer, and the like.
Since then, research and development of organic electroluminescence devices has been thriving. However, light emitting life of the organic electroluminescence device is short for use in displays necessitating a long life, and sufficient life characteristics are not able to be obtained. Therefore, to improve life characteristics, organic materials (organic EL materials) used in the organic layer, particularly a hole transport material used in the hole transport layer, are being studied. Specifically, a technology that uses a carbazole derivative as the hole transport material has been proposed.
As a result of such research on organic EL materials, life characteristics sufficient for use as displays for mobile phones and MP3 players and displays, such as a viewfinder of a camcorder, are able to be ensured. As a result, commercialization as displays for use in on-vehicle audio equipment and mobile devices is advancing. However, display life for use as television displays, such as cathode ray tube (CRT) and liquid crystal displays, has not been reached. Therefore, organic electroluminescence devices with further improved life characteristics for use in household display units replacing CRT, plasma displays, and liquid crystal displays are being developed.
A top emission type organic electroluminescence device is known as an organic electroluminescence device used in the foregoing display unit. Specifically, as illustrated in FIG. 4, for example, a top emission type organic electroluminescence device 101 has a structure in which a light-reflective anode 103, an organic layer 104, and a light-transmissive cathode 105 are sequentially layered on a drive substrate 102. The drive substrate 102 has a drive circuit, such as a thin film transistor (TFT). The organic layer 104 includes a hole transport layer 104A, a light emitting layer 104B, and an electron transport layer 104C in sequence from the anode 103 side. Therefore, emitted light is able to be extracted from the other side (cathode 105 side) of the drive substrate 102, thereby being advantageous for improving an aperture ratio of a light emitting section. Improvement of the aperture ratio enables sufficient light emission intensity to be obtained even in the case where current density applied to the organic layer 104 is reduced, thus leading to improvement in life characteristics.
The anode in such top emission type organic electroluminescence device is composed of a high-reflectance material to efficiently extract emitted light from the cathode side. An example of the material is aluminum. However, aluminum has a relatively small work function. Therefore, sufficient injection of holes into the organic layer is difficult and favorable light emitting efficiency is difficult to obtain. Thus, in compensation, use of an aluminum alloy is proposed (see, for example, Japanese Unexamined Patent Application Publication No. 2003-234193) in which the aluminum alloy contains from 20% to 30%, both inclusive, metal having a high work function, such as copper, palladium, gold, or nickel, as an accessory component metal.
As the materials composing the anode, use of aluminum as a main component and neodymium or the like that is less expensive than palladium and the like and has a relatively smaller work function than that of aluminum as the accessory component metal is also proposed (see, for example, Japanese Unexamined Patent Application Publication No. 2006-079836). In this case, an azatriphenylene or triphenylene derivative is used as the material composing the hole injection layer provided on the anode. However, since materials having hole injection characteristics have strong crystalline, a microcrystalline thin film is easily formed. As a result, peeling and infiltration of moisture occur. A dark spot tends to grow if the organic electroluminescence device is driven for a long period, causing disadvantages in reliability.
Each layer forming the organic layer is ordinarily formed by a vapor phase deposition method, such as vacuum evaporation method. In this case, for example, if foreign matter is present on the anode, the organic layer is formed over the foreign matter. Therefore, an area that is not covered (uncovered section) is formed on the anode. When the cathode is provided on the organic layer in a state in which the uncovered section is formed, the anode and the cathode short-circuit in the uncovered section. Light is not emitted in a pixel where the short circuit occurs, thereby further reducing reliability.
Therefore, to solve the foregoing disadvantages, formation of a mixed layer as a hole injection layer by a coating method is proposed in which, in the mixed layer, an aniline derivative is doped with an electron acceptor substance (see, for example, Japanese Unexamined Patent Application Publication No. 2002-151272).